The present invention relates to a pneumatic tire improved in tire performances, such as wear resistance, uneven wear resistance, fuel economy and the like, all in a good balance.
Tires of radial structure in which the carcass cords are arranged almost at the right angle to the equatorial plane of the tire have been widely used because of their superior wear resistance and steering stability.
Such radial tires are provided with the hoop effect by disposing outside the carcass a so-called belt formed by arranging organic or inorganic cords at relatively small angles to the equatorial plane of the tire and thereby having a relatively large stiffness.
On the other hand, the shape of the radial tire has been designed in a spontaneously balanced shape, by which the carcass profile is not changed from that in the vulcanizing mold when the tire is inflated to a standard internal pressure.
Here, the spontaneously balanced shape means a carcass profile obtained by the theory of spontaneously balanced shape which was proposed by W. Hofferberth in Kautsch. Gummi (8-1955, 124 to 130).
In this theory, the belt located in the tread portion of the tire is regarded as a stiff ring body which does not change its shape when the internal pressure is increased, and it is intended to preliminarily form the carcass extending between this belt and the bead cores which also does not change its shape, by using a vulcanizing mold, with a shape which generates no deformation when the tire is inflated.
The carcass profile based on the theory of spontaneously balanced shape is formed with the intention of causing no deformation on the carcass when the tire is inflated as described above, that is, causing the tensile force to act evenly on the carcass cord.
Though the Hofferberth's theory related to bias tires, it was developed by Akasaka in "The sectional shape of the radial tire," Japan Composite Material Magazine Vol.3,4 (1977), 149 to 154, that the theory also be applied to radial tires.
For application of this theory of the spontaneously balanced shape, it is desired to understand at least the following two facts.
In the first place, the belt is not a perfect stiff body in practice, even when composed of metallic cords, but is slightly deformed by the application of internal pressure, and in particular, the smaller the tire aspect ratio is, the more the belt is likely to be deformed by push-up of the carcass due to the inflation with internal pressure.
In the second place, the bead portion is stiffer due to the turnup part of the carcass, bead apex and other reinforcing layers, and the theory of spontaneously balanced shape cannot be applied in the region from the bead core to the inflexion point of the carcass profile generally called the rim point, that is, the equivalent bead position, and therefore, the curve based on the theory should be considered to start from the equivalent bead position.
In such radial tires having the carcass profile depending on the theory of the spontaneously balanced shape, especially tires in which the carcass profile is closer to a circle because of its relatively high aspect ratio, and even when the tire is inflated to the standard internal pressure, the radially outward deformation of the carcass 6 between the carcass 6 and the belt 7 at the shoulder in particular near the edge of the belt 7, is small as shown in FIG. 8. In the theory of spontaneously balanced shape, the carcass is assumed to have no elongation due to the inflation, but in practice, the carcass slightly extends and the carcass profile based on the theory of spontaneously balanced shape expands to a roughly similar figure, so that the bonding force between the carcass 6 and the belt 7 by pressing against each other cannot be so large, and therefore the constraint between the carcass 6 and the belt 7 is inferior.
In consequence, the tension acting on the carcass 6 hardly works on the belt 7, particularly around its belt edges, and the belt 7 cannot exert its hoop effect satisfactorily. Then the tread profile in the shoulder region tends to become uneven, and together with the fact that the constraint by the belt 7 falls down, an uneven wear is generated as the tires are driven, thereby decreasing the wear resistance.
Above all, in high aspect ratio tires, there is likely to be generated spot wear in which the tread is locally worn at portions with a small ground contact pressure and a large slipping with the road.
In addition, because the amount of the bend of belt in the radial direction while rolling is increased, the stress acting on the belt 7 is also increased, and the rolling resistance is raised, thereby worsening the fuel consumption.
In the Japanese Patent Application No. 61-252465, the present applicant proposed a radial tire for passenger cars which is improved in wear resistance, uneven wear resistance and fuel economy by increasing the amount of the radially outward movement of the shoulder portions when inflated to the standard pressure so as to enlarge the radius of curvature of the tread and thereby to act the tension of the carcass 6 effectively on the belt 7 and to increase the hoop effect of the belt 7. Incidentally, a manufacturing method of such radial tires for passenger cars was proposed in the specification and drawings of Japanese Patent Application No. 62-13408.
The present invention was completed after a further study on the wear resistance and fuel consumption and other factors by paying attention mainly to the amount of the radially outward movement of the tread caused at various points by tire inflation.
It is therefore a primary object of the present invention to provide a pneumatic tire, in which, by the inflation to the standard internal pressure, the tread profile is changed into a preferable shape at respective points, and the above mentioned tire performances are widely improved in comparison with prior art tires.
As the amount of movement at the shoulder portions of this tire is large, the hoop effect is enhanced at these portions to increase the constraint by the belt, thereby exerting the above improving effects.
Consequently, though the ground contacting shape in the prior art tires becomes almost circular, as shown in FIG. 4, and the portion K with a large ground contact pressure exists in the crown centered on the equator of the tire, the ground contacting face of the tire according to the present invention becomes large in area and almost rectangular in shape, and the portion K, K with a large ground contact pressure exist at the side edges, that is, the shoulder regions, and the above effects were thereby brought about.
In order to further improve tire performances, it was learned to preferably maintain an appropriate strong ground contact pressure at the crown region as well as to increase the ground contact pressure at the shoulder regions, and accordingly to roughly even the ground contact pressure in the tire.
Therefore, from the viewpoint of making uniform the ground contact pressure, the present inventors continued studying on the tread profile and the tread thickness at various positions, and then they reached the conclusions that, by designing the tread profile in a so-called double-radius profile in which the tread face is composed of arcs having different radii of curvature between the crown region and the shoulder regions, there can be formed tires having a relatively wide ground contact face of a roughly octangular shape and locating the portion K with a large ground contact pressure at the crown region as shown in FIG. 6, and that, combining the tread face showing the ground contact face of the FIG. 6 shape and that showing the FIG. 5 shape, the distribution of the ground contact pressure can be made uniform as shown in FIG. 7, and thereby the tire performances can be further improved.
It is therefore, another object of the present invention to provide a highly improved pneumatic tire in which the hoop effect is enhanced at the shoulder regions and at the same time the ground contact pressure in the ground contact face is uniforme.